Twin brothers will attempt three marathons in three days along the Dorset coast to raise money for their disabled friend.
Matt and Simon Phelps, from Poole, will undertake the Jurassic Coast Challenge in March, to help pay for "much needed" physio for Rob Ayres.
Mr Ayres, 27, from Leicester, suffered a brain injury in 2008 when he was hit by a car on a night out.
Matt said: "He couldn't talk or move his limbs. Now he aims to walk again."
Before he was injured Mr Ayres had served two tours in Iraq with the Royal Marines, before leaving to embark on a career as a nurse.
Matt said: "When Rob was initially taken to hospital his family and girlfriend were told the devastating news that it was unlikely he'd last the night.
"I was absolutely gutted, I didn't think I'd get the chance to see him again.
"Of course being a fighter Rob survived, but then doctors said he would never talk again. A year later though and he uttered his first words.
"It was thought he would never be able to move his limbs properly again, but after dedication and hard graft he has function in them and is now learning to walk."
Mr Ayres was discharged from hospital recently, after two years.
Matt added: "Unfortunately this now means that his family have to fund his neuro-physio appointments, which cost £90 an hour, and it's likely he'll need at least one a week for the foreseeable future.
"If Rob is ever to achieve his goal of walking again these appointments are imperative."
Matt and Simon have been training for the 78.6-mile (126.49km) challenge, to help fund some of these costs, since December 2010.
The brothers are currently running 40 to 50 miles (64 to 80km) a week in preparation, through Sandbanks, Lulworth and Swanage, and have so far raised about £700 for Mr Ayres.
Matt said: "The Jurassic Coast is such an inspiring landscape it almost makes running a pleasure.
"It's nice to be able to get out in the fresh air instead of having to run in the streets."
Although training is going well Matt admits to a few "minor drawbacks".
He said: "I'm not a natural runner, I've got flat feet.
"And Simon has already got shin splints, and he had to go to the shop mid-run the other morning to buy plasters for his chafed nipples."